As the number of school shootings continues to grow, topped by last week's tragedy in San Diego, several patterns have emerged: The shooters were often bullied. Many told classmates about their plans ahead of time. And - as in the case of alleged 15-year-old shooter Andy Williams - the vast majority got their guns from their own homes or that of a relative.

As a result, new questions are being raised about the presence of firearms in the home, and the potential moral and legal liability of parents when their kids use those guns to kill.

"We are a society in love with our guns," says Robert Meyers, a professor of anthropology and public health at Alfred University in New York. "We need to get a grip on how we are socializing our children into that, as well as examine what are the responsibilities of parents in restricting access to weapons in the home."

Kids' access to firearms is one of the factors examined in a study of school shootings by the US Secret Service National Threat Assessment Center (NTAC). According to the survey, two-thirds of the 41 students involved in 37 school-shooting incidents since 1974 got their guns from their own home or that of a relative. In some cases, the guns had been gifts to the attackers from their own parents.

Statistics of gun ownership

The sheer statistics of gun ownership in America are staggering. The Coalition to Stop Gun Violence estimates there are 192 million guns in private hands (65 million handguns, 49 million shotguns, 70 million rifles), of which handguns account for 80 percent of all firearm homicides.

The guns are not evenly distributed. Fewer than 10 million individuals own well over half the total figure, with 34 percent of gun owners possessing four or more firearms.

But to many, the issue of how those guns are kept is far more sobering.

The American Journal of Public Health last summer said 43 percent of American homes with both guns and children had at least one unlocked firearm - a gun that was not locked away or had no trigger lock. And nearly 10 percent of all gun owners keep their firearms unlocked and loaded.

Thus, a total of 13 percent of American homes with children and guns (1.4 million homes with 2.6 million children) store firearms in a manner accessible to children.

"What has changed in America is availability of guns," says Laurence Steinberg, a criminologist at Temple University in Philadelphia. "In earlier generations, the same sets of problems leading youths to commit these atrocious killings would lead to fist or knife fights. Now they have access to handguns and automatic weapons so the crimes they commit have escalated out of control."

Changes in society

But to blame school shootings on the easy accessibility of guns is to ignore a whole range of societal factors that led the perpetrators to act out in the first place, say others. And in many cases, the shooters might have found a way to obtain a weapon, regardless of whether their parents had one in the home.

Gun groups such as the National Rifle Association (NRA) hold that changes in society are responsible for the increase in such events over earlier decades.

"There are more laws on the books today restricting the use and availability of firearms than 10 and 20 years ago, and yet such shooting incidents are happening that were unheard of then," says Trish Gregory, spokeswoman for the NRA. Fifteen states, including California, require firearms in the home to be locked away, she says, but despite these laws, the number of homicides by young people has continued to grow.

In the past, "it was common in several states for kids to carry rifles and handguns on the school bus for use in target practice after school," says Ms. Gregory. "We think there has been a change in the moral fiber of youth that is leading to this."

But the Coalition to Stop Gun Violence charges that the availability of guns is a clear factor. Handguns in the home increase the likelihood of suicide by a factor of five, says spokesman Desmond Riley, and increase the likelihood of homicide threefold.

"The events of San Diego and other recent school shootings prove this," says Mr. Riley.

Still, the authors of the Secret Service study, as well as many leading experts, say the issue is more complex. They say a whole host of factors from societal mores and community values to family life and entertainment make drawing any conclusions difficult.

"You can find isolated cases to support every cause and position on gun ownership and availability," says Dewey Cornell, a professor at the University of Virginia in Charlottesville. "In addition to issues of access is how our culture values guns as an instrument of power in movies, entertainment, and video - so much so that they are overvalued as a tool by 13- to 15-year-olds who have seen them as a method to solve problems as long as they can remember."

The bullying factor

Helen Smith, a forensic criminologist in Tennessee who works with juveniles in court and has written a book on adolescent violence, says what makes kids kill has nothing to do with the availability of weapons, but rather, is the result of exposure to ridicule, and emotional cruelty.

"Kids that have been pushed beyond the brink will go ahead and make the effort to find whatever weapons they need to carry out their plan," she says.

Ms. Smith's observation is supported by some of the evidence of the NTAC study. The survey looked only at incidents of school attacks in which the school was deliberately chosen as a location important to the shooter, rather than incidents that arbitrarily happened at a school.

"A lot of the post-mortem analyses by press and experts is that these events are caused at the last minute by kids who just snapped and happened to have weapons," says Marissa Reddy, co-author of the study. "But our analysis shows they didn't just wake up one morning and say, 'I am going to take a gun and shoot someone today.' Most had plans for many days, others planned for weeks, months even years."

Relations with parents

Beyond the simple availability of guns, say many experts, it is a meaningful relationship with adults or family that is more important in determining the inappropriate use of firearms.

"There are bad parental relationships running through all these episodes," says Dave Kopel, director of the Independence Institute in Golden, Co.

"When you find parents who are involved in what their kids are up to, it becomes not about who knows how to shoot, who doesn't, who has access, who doesn't, who is socialized into guns or not - it becomes about what it should be: who has a sense of proportion, who has conquered their sense of hatred of others, who has developed tolerance," he says. "Parental responsibility is at the core of this."